Support per-field <access>

[simonschmidt]

Handle cases where <access> is declared inside <field> rather than as a direct child of <register>

For example the stm32f103 SVD has registers like this: https://www.st.com/resource/en/svd/stm32f1_svd.zip

<peripheral>
   ...
  <name>RCC</name>
  <registers>
    <register>
      <name>CR</name>
      ...
      <fields>
        <field>
          <name>HSION</name>
          <access>read-write</access>
          ...

#1 Provides a workaround that treats the whole register as read-write